Heat exchanger design for improved performance and manufacturability

ABSTRACT

A parallel flow heat exchanger is disclosed having heat transfer tubes with a plurality of relatively small channels, which are aligned in a parallel manner, and wherein the heat transfer tubes are in fluid communication with at least one manifold structure, are received in manifold wall openings and are attached to the manifold structure by brazing process The manifold walls and/or the tubes are modified to minimize the likelihood of brazing material plugging or at least partially blocking any of the plurality of channels In one feature, the openings in the manifold structure are formed by deforming the material of the manifold structure outwardly In another feature, the edges of the heat transfer tubes may be formed such that the outermost end channels within each heat transfer tube extend farther inwardly than do the central channels Various design configurations are disclosed.

This application is a United States National Phase application of PCTApplication No. PCT/US2006/049299 filed Dec. 26, 2006.

BACKGROUND OF THE INVENTION

This application relates to a parallel flow heat exchanger, whereinparallel tubes are configured and mounted in a manifold in a manner thatminimizes brazing material blocking channels in the tubes.

Refrigerant systems utilize a refrigerant to condition a secondaryfluid, such as air, delivered to a climate controlled space. In a basicrefrigerant system, the refrigerant is compressed in a compressor, andflows downstream to a heat exchanger (a condenser for subcriticalapplications and a gas cooler for transcritical applications), whereheat is typically rejected from the refrigerant to ambient environment,during heat transfer interaction with this ambient environment. Thenrefrigerant flows through an expansion device, where it is expanded to alower pressure and temperature, and to an evaporator, where during heattransfer interaction with another secondary fluid (e.g., indoor air),the refrigerant is evaporated and typically superheated, while coolingand often dehumidifying this secondary fluid.

In recent years, much interest and design effort has been focused on theefficient operation of the heat exchangers (e.g., condensers, gascoolers and evaporators) in the refrigerant systems. One relativelyrecent advancement in the heat exchanger technology is the developmentand application of parallel flow, or so-called microchannel orminichannel, heat exchangers (these two terms will be usedinterchangeably throughout the text), as the condensers and evaporators.

These heat exchangers are provided with a plurality of parallel heattransfer tubes, typically of a non-round shape, among which refrigerantis distributed and flown in a parallel manner. The heat transfer tubesare orientated generally substantially perpendicular to a refrigerantflow direction in the inlet, intermediate and outlet manifolds that arein flow communication with the heat transfer tubes. The primary reasonsfor the employment of the parallel flow heat exchangers, which usuallyhave aluminum furnace-brazed construction, are related to their superiorperformance, high degree of compactness, structural rigidity andenhanced resistance to corrosion.

In many cases, these heat exchangers are designed for a multi-passconfiguration, typically with a plurality of parallel heat transfertubes within each refrigerant pass, in order to obtain superiorperformance by balancing and optimizing heat transfer and pressure dropcharacteristics. In such designs, the refrigerant that enters an inletmanifold (or so-called inlet header) travels through a first multi-tubepass across a width of the heat exchanger to an opposed, typicallyintermediate, manifold. The refrigerant collected in a firstintermediate manifold reverses its direction, is distributed among theheat transfer tubes in the second pass and flows to a secondintermediate manifold. This flow pattern can be repeated for a number oftimes, to achieve optimum heat exchanger performance, until therefrigerant reaches an outlet manifold (or so-called outlet header).Obviously, in a single-pass configuration, the refrigerant travels onlyonce across the heat exchanger core from the inlet manifold to theoutlet manifold. Typically, the individual manifolds are of acylindrical shape (although other shapes are also known in the art) andare represented by different chambers separated by partitions within thesame manifold construction assembly.

Heat transfer corrugated and typically louvered fins are placed betweenthe heat transfer tubes for outside heat transfer enhancement andconstruction rigidity. These fins are typically attached to the heattransfer tubes during a furnace braze operation. Furthermore, each heattransfer tube preferably contains a plurality of relatively smallparallel channels for in-tube heat transfer augmentation and structuralrigidity.

In the prior art, the openings to receive the multi-channel tubes areformed in a manifold wall by punching the wall inwardly. The heattransfer tubes are inserted into these openings, but do not extend muchfurther into the manifold past the ends of the punched material, sinceit would create additional impedance for the refrigerant flow within themanifold, promote refrigerant maldistribution and degrade heat exchangerperformance. Since the heat transfer tube edges are located atapproximately the same positions as the ends of the punched material ofthe manifold openings, brazing material has a high potential of flowinginto some of the channels during the brazing process and blocking thesechannels. This is, of course, undesirable and should be avoided, sinceat least partially blocked heat transfer tubes are not utilized to theirfull heat transfer potential, have additional hydraulic resistance onthe refrigerant side and promote refrigerant maldistribution conditions.All these factors negatively impact heat exchanger performance.

SUMMARY OF THE INVENTION

In one disclosed feature of this invention, the heat exchanger manifoldopenings for insertion of heat transfer tubes are punched outwardly ofthe manifold wall. Therefore, the heat transfer tubes can be insertedinto the openings, and extend just slightly beyond the wall of themanifold, and far beyond the manifold opening ends, such that channelsin the heat transfer tubes are unlikely to be blocked by brazingmaterial during the brazing process. Moreover, a relatively graduallycurved interface is formed between the manifold openings and the heattransfer tube edges to serve as a well to receive the brazing material.

In a separate feature of this invention, the shape of the heat transfertube edges is varied such that it is not a straight line, but is ratherrepresented by a shape that closely follows and resembles the curvatureof the manifold wall. For instance, the heat transfer tube edges canhave a circular shape, piecewise circular shape, elliptical shape, etc.or have a triangular cutout, rectangular cutout, trapezoidal cutout,etc. Many variations and combinations of these basic shapes are feasibleand within the scope of the invention. In this manner, the heat transfertubes can extend beyond the punched material of the heat exchangermanifold openings without blocking refrigerant flow, as they have thedesigned-in recesses in the center channels allowing the end channels ofheat transfer tubes penetrate further into the manifold. Therefore, theend channels, that are most likely to be plugged by the brazing materialduring the brazing process, can extend farther into the manifold beyondthe manifold opening ends. This eliminates channel blockage by thebrazing material, while not introducing any additional undesiredhydraulic impedance to the refrigerant flow in the manifold. As aresult, refrigerant maldistribution conditions are avoided, the entireheat transfer surface is fully utilized, pressure drop through the heatexchanger is reduced and the heat exchanger performance is improved.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a refrigerant system.

FIG. 2 is a cross-sectional view of a parallel flow heat exchanger.

FIG. 3A shows a feature of the prior art manifold assembly.

FIG. 3B shows a top view of the prior art manifold assembly shown inFIG. 3A.

FIG. 3C shows the prior art heat transfer tube with end channels blockedby the brazing material.

FIG. 4 shows one inventive feature.

FIG. 5 shows a first embodiment of a second inventive feature.

FIG. 6 shows a second embodiment of the second inventive feature.

FIG. 7 shows a third embodiment of the second inventive feature.

FIG. 8 shows a fourth embodiment of the second inventive feature.

FIG. 9 shows a fifth embodiment of the second inventive feature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A basic refrigerant system 20 is illustrated in FIG. 1 and includes acompressor 22 delivering refrigerant into a discharge line 23 leading toa heat exchanger (a condenser for subcritical applications and a gascooler for transcritical applications) 24. The heat exchanger 24 is aparallel flow heat exchanger, and in one disclosed embodiment, is amicrochannel heat exchanger. The heat is transferred in the heatexchanger 24 from the refrigerant to a secondary loop fluid, such asambient air. The high pressure, but cooled, refrigerant passes into arefrigerant line 25 downstream of the heat exchanger 24 and through anexpansion device 26, where it is expanded to a lower pressure andtemperature. Downstream of the expansion device 26, refrigerant flowsthrough an evaporator 28 and back to the compressor 22. The evaporator28 is a parallel flow heat exchanger, and in one disclosed embodiment,is a microchannel heat exchanger. Although a basic refrigerant system 20is shown in FIG. 1, it is well understood by a person ordinarily skilledin the art that many options and features may be incorporated into arefrigerant system design. All these refrigerant system configurationsare well within the scope and can equally benefit from the invention.

The parallel flow heat exchanges 24 and 28 may have a single-passconfiguration or a multi-pass configuration. A single-pass configurationis more typical for the parallel flow evaporators, while a multi-passconfiguration is frequently used for the parallel flow condensers andgas coolers. Although FIG. 2 depicts an exemplary embodiment of amulti-pass (5-pass) parallel flow condenser or a gas cooler, as known toa person ordinarily skilled in the art, many design variations ofparallel flow heat exchangers are feasible and would be within the scopeof the invention. As shown in FIG. 2, the multi-pass parallel flowcondenser or gas cooler 24 has a manifold structure 30 that consists ofmultiple chambers 30A, 30B, and 30C, as well as a manifold structure 34that consists of multiple chambers 34A, 34B, and 34C, and positioned atan opposite end of the heat exchanger core. The inlet manifold chamber30A receives the refrigerant from the discharge line 23. The refrigerantflows into a first bank of parallel heat transfer tubes 32, and thenacross the heat exchanger core to the intermediate manifold chamber 34A.From the intermediate manifold chamber 34A, the refrigerant flowsthrough a second bank of parallel heat transfer tubes 132, in anopposite direction, to the intermediate manifold chamber 30B. In asimilar manner, the refrigerant flows between the intermediate manifoldchambers 30B and 34B, through a third bank of parallel heat transfertubes 232, and between the intermediate manifold chambers 34B and 30C,through a forth bank of parallel heat transfer tubes 332. Finally, fromthe intermediate manifold chamber 30C, the refrigerant flows to theoutlet manifold chamber 34C, through a fifth bank of parallel heattransfer tubes 432, and to the refrigerant line 25. It should be notedthat, in practice, there may be more or less refrigerant passes than theillustrated passes 32, 132, 232, 332, and 432. Further, it should beunderstood that, although for simplicity purposes each refrigerant passis represented by a single heat transfer tube, typically, there are manyheat transfer tubes within each pass amongst which refrigerant isdistributed while flowing within the pass. In the multi-pass condenserand gas cooler applications, a number of the parallel heat transfertubes within each bank typically decreases in a downstream direction,with respect to a refrigerant flow. On the other hand, in the multi-passevaporator applications, a number of parallel heat transfer tubes ineach bank generally increases in a downstream direction, with respect toa refrigerant flow. Separator plates 38 are placed within the manifoldstructures 30 and 34 to separate the chambers 30A, 30B, 30C and thechambers 34A, 34B, and 34C respectively. Obviously, in single-passparallel flow heat exchanger configurations, manifold structures 30 and34 would have only single chambers, in particular, the inlet chamber 34Awithin the manifold structure 30 and the outlet chamber 34C within themanifold structure 34.

As shown in FIG. 3A, in the prior art, there has been a problemassociated with positioning and brazing the heat transfer tubes 32 (aswell as heat transfer tubes 132, 232, 332, and 432) into the manifoldstructure 30 (as well as into the manifold structure 34). As shown,manifold openings 40 for receiving the heat transfer tubes 32 are formedby punching the material of the wall of the manifold 30 inwardly. Thismakes a portion of material 43 for the manifold openings extending intothe flow passage within the manifold structure 30. A brazing material 42is then positioned between the material of the heat transfer tubes 32and the manifold material 43, and secures the heat transfer tubes 32within the manifold structure 30, during a brazing process. A problemcan occur with this prior art design, as is shown in FIG. 3B. As shownin FIG. 3B, the heat transfer tube 32 has a plurality of relativelysmall channels (so-called microchannels or minichannels) 44 that arealigned in a parallel manner into the plane of the paper in the FIG. 3Aview. Internal walls or fins 45 separate the small parallel channels 44.The fins 45 are placed between the channels 44 for structural rigidityand heat transfer enhancement. Such microchannel or minichannel heatexchangers are becoming more widely utilized in the air conditioning andrefrigeration art and beyond. However, in the conventional interfacedesign between the heat transfer tubes 32 and the manifold structure 30shown in FIG. 3B, the outermost end channels 46 can be blocked by thebrazing material 42, since the edges of the heat transfer tubes 32 arerelatively close to the forward ends of the punched material 43 of themanifold openings 40. Thus, as shown schematically at FIG. 3C, theoutermost channels 46 may become at least partially blocked or pluggedwith the brazing material 42. This is undesirable, since it would createadditional impedance for the refrigerant flow through the heat transfertubes, reduce heat transfer due to only partial utilization of the heattransfer surface, promote refrigerant maldistribution conditions anddegrade the heat exchanger performance. Extending the heat transfertubes 32 farther inside the manifold 30 is also undesirable, sinceadditional refrigerant pressure drop within the manifold 30 andpotential refrigerant maldistribution make a negative impact on the heatexchanger performance.

FIG. 4 shows a first feature of the present invention. In FIG. 4, themanifold openings 54 are formed by deforming material of the wall 56 ofthe manifold 50 outwardly. Now, the heat transfer tubes 32 may havetheir edges 58 just slightly extending inwardly of the wall of themanifold 50, but positioned farther away from the edges of the manifoldopenings 54. The brazing material 52 is at the interface locations,between the manifold openings 54 and the heat transfer tube edges 58,that is gradually curved away from the heat transfer tube edges 58, andthus is positioned in a well or cavity. The edges 58 of the heattransfer tubes 32 minimally extend inwardly of the manifold 50 withoutunduly blocking refrigerant flow within the manifold. Thus, the problemsas mentioned above are addressed by this feature.

Other modifications to the heat transfer tube provide further relieffrom the likelihood of brazing material blocking the channels. Thefeatures shown in FIGS. 5-8 may be utilized in conjunction with, or inplace of, the features shown in FIG. 4.

As shown in FIG. 5, the edge of a heat transfer tube 60 can have acurvature that generally follows the manifold cross-section shape, asshown at 62, such that the outermost end channels 46, which are the onesmost likely to be plugged or at least partially blocked with the brazingmaterial, can extend further into the manifold 30 and away from the endsof the manifold openings 68, preventing blockage of these outmost endchannels 46 by the brazing material 64, while the curvature 62 providesa recess in the center section of the manifold 30 that relieves theabstraction to the refrigerant flow within the manifold, as mentionedabove. For instance, the heat transfer tube edge 62 can be of a circularshape, a piecewise circular shape, an elliptical shape or any othershape having a curvature.

Analogously, FIG. 6 shows a heat transfer tube 70 having a triangularcutout 72 at the edge that provides similar benefits to the curvature 62of FIG. 5 embodiment.

FIG. 7 shows a heat transfer tube 80 having a rectangular cutout 82providing the same function.

FIG. 8 shows a tube 90 having a trapezoidal cutout 92 that providessimilar functionality to the FIG. 5-7 embodiments.

It should be noted that any combination of the FIG. 5-8 embodiments isalso within the scope of the invention.

Also, heat transfer tubes of other shapes or cross-sections can benefitfrom the invention. For instance, as shown in FIG. 9, a round tube 102having internal heat transfer enhancement elements 104 can takeadvantage of the invention, in a similar manner. Furthermore, theinvention extends to other manifold shapes and cross-sections. Lastly,the invention offers similar benefits in other applications, outside thescope of air conditioning and refrigeration art, where any other fluidcan flow inside the channels of parallel heat transfer tubes. Lastly,any other manufacturing process utilizing the material, such as, forinstance, solder or glue, securing the heat transfer tubes to themanifold, that is initially fluent and then solidifies, during thisattachment manufacturing process, can equally benefit from theinvention.

In summary, the present invention provides a variety of ways to minimizethe blockage of channels in microchannel heat exchangers by the brazingor other securing material, resulting in avoiding refrigerant (or otherfluid) maldistribution conditions, entire heat transfer surfaceutilization, in-tube pressure drop reduction through the heat exchangerand improved heat exchanger performance.

While preferred embodiments of this invention have been disclosed, aworker of ordinary skill in the art would recognize that certainmodifications would come within the scope of this invention. For thatreason the following claims should be studied to determine the truescope and content of this invention.

1. A heat exchanger comprising: a pair of spaced manifold structures,and a plurality of heat transfer tubes extending between said manifoldstructures in generally parallel relationship with each other and beingin fluid communication with said manifold structures, each of said heattransfer tubes having a plurality of parallel channels spaced from eachother, and said heat transfer tubes being inserted in openings in saidmanifold structures, said heat transfer tubes being secured to saidmanifold structures by an initially fluent and then solidifying securingmaterial, and there being modifications to at least one of said manifoldstructures and said heat transfer tubes to minimize the likelihood ofsaid securing material at least partially blocking any of said pluralityof channels; and a working fluid to flow inside said heat transfer tubesis one of a refrigerant, air, water, glycol solution, oil, air,nitrogen, helium, petrochemical gas and combination thereof.
 2. The heatexchanger as set forth in claim 1, wherein said securing material is oneof brazing material, solder material and glue material.
 3. The heatexchanger as set forth in claim 2, wherein said securing material isdeposited within an internal passage in said manifold structures tosecure said heat transfer tubes within said manifold structure.
 4. Theheat exchanger as set forth in claim 1, wherein said openings are formedin said manifold structures by deforming the material of said manifoldstructures outwardly away from an internal passage in said manifoldstructures such that said heat transfer tubes do not extend inwardly ofsaid manifold structures passing farther beyond a wall of said manifoldstructures.
 5. The heat exchanger as set forth in claim 1, wherein edgesof said heat transfer tubes are shaped such that laterally outermostchannels of said plurality of parallel channels extend inwardly fartherbeyond said manifold walls then do more centrally located channels ofsaid plurality of parallel channels.
 6. The heat exchanger as set forthin claim 5, wherein edges of said heat transfer tubes are shaped to haveone of a triangular cutout, a rectangular cutout and a trapezoidalcutout such that the laterally outermost channels of said plurality ofparallel channels extend farther inwardly passing beyond said manifoldwalls than centrally located channels of said plurality of parallelchannels.
 7. The heat exchanger as set forth in claim 1, wherein edgesof said heat transfer tubes are shaped to have a curvature such that itgenerally follows and resembles a manifold curvature.
 8. The heatexchanger as set forth in claim 1, wherein said heat transfer tube edgeshave a curvature of one of a circle and an ellipse.
 9. A heat exchangercomprising: a pair of spaced manifold structures, and a plurality ofheat transfer tubes extending between said manifold structures ingenerally parallel relationship with each other and being in fluidcommunication with said manifold structures, each of said heat transfertubes having a plurality of parallel channels spaced from each other,and said heat transfer tubes being inserted in openings in said manifoldstructures, said heat transfer tubes being secured to said manifoldstructures by an initially fluent and then solidifying securingmaterial, and there being modifications to at least one of said manifoldstructures and said heat transfer tubes to minimize the likelihood ofsaid securing material at least partially blocking any of said pluralityof channels; and said heat transfer tube material and said manifoldmaterial is one of copper and aluminum.
 10. The heat exchanger as setforth in claim 9, wherein a working fluid to flow inside said heattransfer tubes is one of a refrigerant, air, water, glycol solution,oil, air, nitrogen, helium, petrochemical gas and combination thereof.11. A refrigerant system comprising: a compressor, a heat rejecting heatexchanger, an expansion device, and an evaporator; and at least one ofsaid evaporator and said heat rejecting heat exchanger including a pairof spaced manifold structures, and a plurality of heat transfer tubesextending between said manifold structures in generally parallelrelationship with each other and being in fluid communication with saidmanifold structures, each of said heat transfer tubes having a pluralityof parallel channels spaced from each other, and said heat transfertubes being inserted in openings in said manifold structures, said heattransfer tubes being secured to said manifold structures by an initiallyfluent and then solidifying securing material, and there beingmodifications to at least one of said manifold structures and said heattransfer tubes to minimize the likelihood of said securing material atleast partially blocking any of said plurality of channels, while theheat exchanger performance is not compromised; and said securingmaterial is one of brazing material, solder material and glue material.12. The refrigerant system as set forth in claim 11, wherein saidsecuring material is deposited within an internal passage in saidmanifold structures to secure said heat transfer tubes within saidmanifold structure.
 13. The refrigerant system as set forth claim 11,wherein said heat transfer tubes have one of a rectangular, oval,flatten circle, racetrack, elliptical or circular cross-section.
 14. Therefrigerant system as set forth in claim 12, wherein said openings areformed in said manifold structures by deforming the material of saidmanifold structures outwardly away from an internal passage in saidmanifold structures such that said heat transfer tubes do not extendinwardly of said manifold structures passing farther beyond a wall ofsaid manifold structures.
 15. The refrigerant system as set forth inclaim 12, wherein edges of said heat transfer tubes are shaped such thatlaterally outermost channels of said plurality of parallel channelsextend inwardly farther beyond said manifold walls then do morecentrally located channels of said plurality of parallel channels. 16.The refrigerant system as set forth in claim 15, wherein edges of saidheat transfer tubes are shaped to have one of a triangular cutout, arectangular cutout and a trapezoidal cutout such that the laterallyoutermost channels of said plurality of parallel channels extend fartherinwardly passing beyond said manifold walls than centrally locatedchannels of said plurality of parallel channels.
 17. The refrigerantsystem as set forth in claim 11, wherein edges of said heat transfertubes are shaped to have a curvature such that it generally follows andresembles a manifold curvature.
 18. The refrigerant system as set forthin claim 11, wherein said heat transfer tube edges have a curvature ofone of a circle and an ellipse.
 19. The refrigerant system as set forthin claim 11, wherein said heat transfer tube material and said manifoldmaterial is one of copper and aluminum.